The enzyme 5-lipoxygenase catalyzes the first step in the formation of leukotrienes. These compounds exhibit a broad range of biological effects including chemokinetic and chemotactic activity towards granulocytes, bronchospastic action, induction of microvascular permeability leading to plasma leakage and stimulation of airway mucous secretion. The use of 5-lipoxygenase inhibitors and leukotriene receptor antagonists have linked the leukotrienes and their actions to the pathophysiology of inflammatory disorders such as allergic rhinitis, asthma and bowel disease, but non-specific drug effects and residual leukotriene synthesis have prevented a clear picture of the role of leukotrienes in health and disease. We propose to undertake experiments to clarify the role of the 5-lipoxygenase pathway in mice by using the technique of homologous recombination to completely abrogate gene function and resultant leukotriene formation. 5-Lipoxygenase gene expression will be studied in detail in normal mice and the gene and chromosomal locus characterized. A 5-lipoxygenase targeting construct will be inserted into embryonic stem cells and mice carrying the inactivated gene will be generated. Homozygous null allele mice will be investigated fore the phenotypic consequences of 5-lipoxygenase gene inactivation and several myeloid cell types will be characterized with respect to biochemical and functional parameters. Finally, several murine in vivo inflammatory and anaphylactic models will be tested for the involvement of 5-lipoxygenase products. We expect these studies to give a complete and clear insight into 5-lipoxygenase gene function within a single, defined species.